Warped UV for simple model
When you are UV unwrapping, you're basically trying to flatten out a box. You can cut seams around all six faces, but that leaves separated parts. Try to find the edges where the geometry has to spread and mark those as seams, like this:

You can see I made seams down the 45 degree bevels, and just cut the bottom out separately.
After removing all the extra geometry it becomes much easier to achieve the UV layout you wanted: